Phenol compounds with alkylated ortho-position are useful as various industrial materials. For example, 2,6-xylenol is used as a raw material for polyphenylene ether.
A method has already been known to alkylate the ortho-position of a phenol compound by catalytically reacting the phenol compound with an alcohol in the gaseous phase. In such a method, however, when it is desired to obtain, for example, 2,6-dialkyl phenol, para-substituted products such as a para-alkyl phenol, 2,4-dialkyl phenol and 2,4,6-triaklyl phenol are formed as by-products. These by-products are difficult to separate from the 2,6-dialkyl phenol, and therefore, it is important to inhibit the formation of these by-products as much as possible by raising the selectivity of the reaction.
The conventional processes are all disadvantageous for commerical operation because the selectivity is low or the yield of the object product is low. Furthermore, they have the defect in that the durability of the catalytic activity is low and therefore the catalyst should be exchanged frequently.
For example, U.S. Pat. No. 2,448,942 discloses a process for methylating a phenol compound using a metal oxide, especially aluminum oxide, as a catalyst. However, this process is suitable for obtaining a phenol compound which is substituted with three or more alkyl groups, and the selectivity of the alkylation of ortho-position is low.
British Pat. No. 717,588 discloses a process for preparing 2,6-xylenol by alkylating ortho-cresol with methanol using a metal oxide, preferably aluminum oxide, as the catalyst. However, according to this process, the conversion of ortho-cresol and the selectivity to 2,6-xylenol are very low.
There was another proposal for making 2,6-xylenol by methylating phenol using magnesium oxide as a catalyst. Since this process requires high temperatures, the catalytic activity gradually decreases by the deposition of a carbonaceous substance on the catalyst surface and the growth of the crystallites of magnesium oxide causes an irreversible decrease in its activity, which in turn leads to the necessity of exhanging the catalyst after a short period of time.
It has also been known to use iron oxide as a catalyst, but since it inherently has a low catalytic activity and the activity decrease abruptly, this method has not been utilized commercially. In an attempt to remove these defects of the iron oxide catalyst, method has been proposed in which a mixture of iron oxide and zinc oxide is used as a catalyst (Japanese Patent Publication No. 37812/71). In this method, too, the activity of the catalyst is not sufficient, and especially its durability is unsatisfactory.
We previously proposed as the catalyst for making an ortho-alkylated phenol with high selectivity, two types of catalyst systems, i.e. binary catalyst consisting of iron oxide and silica, and ternary catalyst consisting of iron oxide, silica and chromium oxide (U.S. Pat. Application Ser. No. 476,268). Of those catalysts, the binary catalyst has excellent initial activity, but also has a defect in that the activity gradually decreases as the reaction time is prolonged. Therefore, the catalyst must be regenerated with oxygen-containing gas at a certain time interval. Whereas, the ternary catalyst has a nearly equivalent initial activity and markedly improved durability compared with the binary catalyst. However, the durability being a very important factor of a catalyst from an industrial standpoint, it is extremely desirable to find a catalyst having still more improved durability than that of the ternary catalyst. An object of this invention is to solve that problem.